Rebel groups across Syria are defying the United States by pledging their allegiance to a group that Washington will designate today a terrorist organization for its alleged links to al-Qaeda.

A total of 29 opposition groups, including fighting “brigades” and civilian committees, have signed a petition calling for mass demonstrations in support of Jabhat al-Nusra, an Islamist group which the White House believes is an offshoot of al-Qaeda in Iraq.

The petition is promoting the slogan “No to American intervention, for we are all Jabhat al-Nusra” and urges supporters to “raise the Jabhat al-Nusra flag” as a “thank you”.

“These are the men for the people of Syria, these are the heroes who belong to us in religion, in blood and in revolution,” read a statement widely circulated on Syrian opposition Facebook pages.

Jabhat al-Nusra made its mark early this year with a string of suicide bombings, a tactic it continues to use. Aided by fighters from abroad and Syrians who have returned from other wars in the Middle East, it has also led battles for a number of military bases and has secured a string of recent victories. Along with allied jihadist groups, it captured the Sheikh Suleiman base west of Aleppo yesterday morning, and has also dented the infrastructure of the regime in the Syrian capital, Damascus.

Although Jabhat al-Nusra remains separate from the Free Syrian Army, many FSA leaders now recognise its strength and order their forces to cooperate with it.